Wood veneer is commonly applied to the outer surfaces of furniture, doors or other architectural woodwork to provide aesthetically pleasing wood grain appearance to such products at a relatively inexpensive cost. Typical base substrate materials can include other veneer sheets, paper, cloth, particleboard, fiberboard, reconstituted wood or fiber based board. The composite structure of a laminate panel constructed of a veneer face bonded to one or a plurality of base substrates will be hereinafter referred to as a “laminated veneer product.” However, thin veneers can show imperfections of the base substrate, such as color show through or pitch bleed. In order to hide the imperfections of the base substrate, a paper backing sheet can be positioned between the veneer and the base substrate.
The manufacture of paper backed veneers typically involves the application of an adhesive resin to at least one surface of the paper backing sheet, followed by adhering the paper backing sheet to the inner facing surface of the veneer. Then, the paper backed veneer can be stacked and pressed to the base substrate(s) to produce a laminated veneer product. The pressing is generally accompanied by heating of the treated veneers in order to accelerate curing of the adhesive, although cold pressing has also been used.
Adhesives known in the art for manufacturing laminated veneer products are predominantly thermosetting adhesives. Aldehydes, especially formaldehyde based resins, are widely used in adhesive compositions. Typically, these adhesives compositions contain a substantial molar excess of formaldehyde. Some of this excess formaldehyde is released upon curing of the resin during the manufacture of the product, which can create environmental concerns in the manufacturing process. Also, the formaldehyde can continue to be released from these products even after the manufacturing process is completed. Thus, formaldehyde in the indoor air has been a major concern for many years. Commonly used aldehyde adhesives include phenol formaldehyde (resole) resins, commonly referred to as PF resins; urea formaldehyde resins (UF resins); melamine formaldehyde resins (MF); resorcinol formaldehyde resins (RF); and various combinations of the foregoing. Phenol formaldehyde (PF) based adhesives are the most widely used, especially for the manufacture of commodity laminated wood products, such as plywood. PF, and related resins types such as UF, RF, and MF, liberate water during the curing process, which limits the moisture content of the veneers that may be used with these kinds of adhesives.
A few formaldehyde-free adhesives have been introduced to the market place in an attempt to make a more environmental friendly adhesive. For example, polyisocyanate adhesives, such as methylene diphenyl diisocyanate, and ethyl carbamate (urethane) resins are formaldehyde-free adhesives that have been used. However, these adhesives are expensive, and thus undesirable for many practical uses of veneer laminate products.
As such, a need exists for a cost-effective, formaldehyde-free free adhesive capable of bonding layers of a veneer product together.